fix sales tax rounding in some edge cases, #42263, from #39487

1 files changed, 9 insertions, 7 deletions

diff --git a/FS/FS/TaxEngine/internal.pm b/FS/FS/TaxEngine/internal.pm
index a9b32d1..db7010c 100644
--- a/FS/FS/TaxEngine/internal.pm
+++ b/FS/FS/TaxEngine/internal.pm
@@ -66,7 +66,7 @@ sub taxline {
   my $taxnum = $tax_object->taxnum;
   my $exemptions = $self->{exemptions}->{$taxnum} ||= [];
   
-  my $taxable_cents = 0;
+  my $taxable_total = 0;
   my $tax_cents = 0;
 
   my $round_per_line_item = $conf->exists('tax-round_per_line_item');
@@ -302,15 +302,17 @@ sub taxline {
     });
     push @tax_links, $location;
 
-    $taxable_cents += $taxable_charged;
+    $taxable_total += $taxable_charged;
     $tax_cents += $this_tax_cents;
   } #foreach $cust_bill_pkg
 
-  # calculate tax and rounding error for the whole group
-  my $extra_cents = sprintf('%.2f', $taxable_cents * $tax_object->tax / 100)
-                            * 100 - $tax_cents;
-  # make sure we have an integer
-  $extra_cents = sprintf('%.0f', $extra_cents);
+  # calculate tax and rounding error for the whole group: total taxable
+  # amount times tax rate (as cents per dollar), minus the tax already
+  # charged
+  # and force 0.5 to round up
+  my $extra_cents = sprintf('%.0f',
+    ($taxable_total * $tax_object->tax) - $tax_cents + 0.00000001
+  );
 
   # if we're rounding per item, then ignore that and don't distribute any
   # extra cents.